The invention relates to a method for the control of the transmission power of a transmitting station in a radio communication system as well as a corresponding transmitting station, a corresponding receiving station and a corresponding radio communication system.
In order to allow an error-free transmission of data for all the data packets to be transmitted in radio communication systems, which are operated for example in accordance with the UMTS standard (UMTS: Universal Mobile Telecommunication System), a method is used whereby a receiving station receives incorrectly received data packets once again. This method is known is known as “Automatic Repeat Request” (ARQ).
If a receiving station, for example a base station, receives an incorrect data packet, i.e. the data packet cannot be decoded correctly, it sends a NACK signal (NACK: Not ACKnowledgement) to a transmitting station, for example, to a subscriber station in order to signal the incorrect reception of a data packet. As a result, the subscriber station transmits the corresponding data packet anew to the base station. The base station sends an ACK signal (ACK: ACKnowledgement) to signal the correct (error-free) reception of a data packet.
If the subscriber station changes to another radio cell, it arrives within the transmitting range and the range of reception of at least one additional base station. In the case of a so-called soft handover, the subscriber station at the same time transmits data packets to the base station of the original cell and to at least one other base station of the new cell. All the data packets received by the base stations are forwarded to a Radio Network Controller [RNC] and evaluated there. In the radio network controller, data packets which have been received correctly, are merged and arranged in the correct order and the same data packets are combined.
The subscriber station receives ACK and NACK signals during a soft handover from at least two of the base stations. A sufficient criterion for transmitting a new data packet is that the subscriber station receives an ACK signal from one of the base stations because for an efficient transmission of data, a data packet needs to be forwarded once error-free to the radio network controller.
In order to guarantee a desired quality of the transmission of data of the subscriber station, an error bit rate (BER: Bit Error Rate or BLER: Block Error Rate) is determined in the radio network controller based on all the received data packets and the error rate is compared with a target value. Depending on the result of this comparison, a target value for a receive quality, for example, a signal-to-interference ratio (SIR: Signal-to-Interference Ratio) is specified for the base stations. On the other hand, the base stations in each case measure a receive quality of the signals received by the subscriber station based on the known control signals (pilot symbols) of the subscriber station and compare the specific measured value with the target value for the receive quality specified by the radio network controller. Depending on the result of the comparison, a signal is sent to the subscriber station to either increase or reduce its transmission power. Signals with a size of 1 bit, so-called TPC bits (TPC: Transmit Power Control) are usually used for this purpose. If the subscriber station receives at least one TPC bit from one of the base stations, which requests a reduction, it will reduce its transmission power in order to generate as few interferences as possible in all the receiving base stations. The regulation of the target value for the receive quality by the radio network controller is called the outer loop power control, while the comparatively fast regulation of the transmission power of the subscriber station is designated by the control signals (pilot symbols) of the base stations as the inner loop power control.
Within the framework of the current standardization projects of the 3GPP (3rd Generation Partnership Project), proposals are being discussed at present to also shift the outer loop power control to the base stations. This is advantageous for a transmission of data with a high data rate designated as the “enhanced uplink” (for this purpose, see 3GPP TR25.896v0.2.0) from a subscriber station to a base station, i.e. the uplink.
In addition, data packets are forwarded in the case of an “enhanced uplink” to the radio network controller and joined there. Redundant data packets are likewise also combined in the radio network controller, which specifies a common error rate, namely, the so-called “active set” to all the receiving base stations. The measurement of the actual error rate and the comparison with the target value for the error rate as well as modifications of the target value for the receive quality are now carried by the base stations themselves.
The target value for the error rate is usually very low (approximately 10−6), so that a measurement of the error rate requires a measuring period of such a length that even slow changes of the transmission conditions can no longer be measured. The target value for the receive quality must therefore be set in such a way that even in the case of poor and rapidly changing transmission conditions, the desired error rate is reached in the radio network controller.
However, if a plurality of base stations now, in the soft-handover, forward at the same time their received data packets to the radio network controller, it is in this way obtained from the above-described setting of the target value for the receive quality that in the radio network controller there is actually a lower error rate than was specified by the target value for the error rate. This is achieved in that, the error rate in the radio network controller is determined by the data packets forwarded from all the base stations and as a result, it is better than from one base station alone. Therefore, the transmission power of the subscriber station is higher than would actually be required and therefore there are more interferences from the data packets transmitted by the subscriber station than would be necessary in a data transmission corresponding with the actual requirements for the error rate.
From EP 1067706 A1, a method for setting a target value for a receive quality is known in which the target value changes depending on a comparison of an averaged frame error rate with a target value of the frame error rate and a comparison of an averaged receive quality with the target value for the receive quality. In addition, it is also known in the case of packet data services to use a plurality of transmission repetitions for packet data units (PDU) instead of the frame error rate.